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This guide gives a repeatable diagnostic process for Chrome and other Selenium WebDriver sessions, explains why “the same window size” can still produce different images, and shows an API alternative when maintaining browser infrastructure is unnecessary.
Why Headless screenshots change
Headless means that no visible browser UI is presented; it does not mean that every rendering input is automatically identical. Selenium controls a browser session, and that session still has a window, a page viewport, a device-pixel ratio, a browser implementation, and a particular moment in the page lifecycle.
Chrome’s current Headless mode and headful mode use the same browser implementation, but the environment around them can differ. Chrome 112 updated Headless to create platform windows without displaying them. From Chrome 132.0.6793.0, the older implementation is distributed as the separate chrome-headless-shell binary. Record the exact Chrome version and which implementation your test launches; “Headless Chrome” is not a sufficient reproducibility description.
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A requested outer-window size is also not the same thing as the CSS viewport. Browser decorations, viewport rules, device scale, and automation settings can make the page’s innerWidth and innerHeight differ from the dimensions you put in a command-line flag. Finally, a screenshot API may capture only the current viewport or a full document. Two images with different heights can therefore be correct outputs from different capture scopes.
No reliable prevalence percentage establishes how often Headless causes differences. Treat every mismatch as an environment-and-capture diagnosis rather than assuming Headless is the sole cause.
The variables to hold constant
For every comparison, save a small “rendering manifest” beside the image. These are the highest-value fields:
| Variable | What to record | Why it matters |
|---|---|---|
| Browser and driver | Exact browser build and matching WebDriver/ChromeDriver versions | Rendering and automation behavior can change between releases. |
| Headless implementation | Modern unified Headless or the standalone chrome-headless-shell |
They are different implementations and must not be treated as interchangeable. |
| Window geometry | Requested size and the observed outer window rectangle | The requested value is configuration, not proof of the resulting size. |
| CSS viewport | innerWidth, innerHeight, and visualViewport values |
This is the layout space your page actually receives. |
| Screen scale | devicePixelRatio and Headless screen scale configuration |
CSS pixels and bitmap pixels can map differently. |
| Capture scope | Current viewport, element, or full document | Scope directly affects image dimensions and visible content. |
| Host and state | Operating-system/container details, URL, data, and readiness condition | Fonts, compositor behavior, and dynamic content can vary outside Selenium’s size settings. |
Keep the first five fields identical where possible, and investigate the remaining fields when the pixels still differ. Matching them improves comparability; it cannot guarantee pixel identity on every host.
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A repeatable diagnostic procedure
-
Log versions and launch arguments
Store the browser version, driver version, operating system or container image, the complete Chrome options, and whether the run uses modern Headless or
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-
Set a deterministic window
Use Selenium’s window-management API or a Chrome startup option. Selenium’s window and tab documentation demonstrates
set_window_sizeand explains why screen resolution can affect rendering.from selenium import webdriver from selenium.webdriver.chrome.options import Options options = Options() options.add_argument("--headless=new") # This is an outer-window request, not a viewport assertion. options.add_argument("--window-size=1280,900") driver = webdriver.Chrome(options=options) driver.set_window_size(1280, 900)Use one mechanism as your source of truth in a test suite. Setting both is useful while diagnosing, but still read back the result.
-
Measure what the session actually received
After navigation, collect the window rectangle and page metrics. The following diagnostic is deliberately explicit:
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-
Control the Headless screen and scale
Chrome’s virtual-screen controls let Headless use an explicit screen size and scale independent of a physically attached display. The documented examples use 800×600 and 600×800 virtual screens; they are examples, not universal standards. See Chrome’s virtual-screen configuration for the flags supported by your browser version.
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Confirm screenshot scope
Selenium’s general Chromium screenshot API documents capture of the current window. Firefox separately exposes a specifically named full-page screenshot API, illustrating that full-document behavior is browser/API-specific. Decide whether your test needs the visible viewport, a selected element, or the entire document, then call an API that explicitly provides that scope. Do not infer scope from the file’s name or height.
-
Wait for visual stability
Wait for the application state your assertion depends on: a selector becoming visible, asynchronous data finishing, images being decoded, fonts loading, and any animations settling. A fixed sleep can be a fallback, but a state-based wait is easier to reason about. Capture a diagnostic image and the manifest at the same point in both runs.
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Chrome command-line examples
Chrome’s command-line documentation pairs an explicit window size with screenshot capture:
chrome --headless --screenshot --window-size=412,892 https://developer.chrome.com/The 412×892 dimensions are a documentation example, not a recommended baseline. The command is useful for proving what Chrome itself does, but a Selenium test should also log the observed rectangle and CSS viewport. Read the Headless command-line reference for options supported by your installed release.
Common symptoms and fixes
The image is wider or narrower
Compare innerWidth, visualViewport.width, and devicePixelRatio first. Set the window explicitly, then verify the values rather than trusting --window-size. Also check whether one run uses a mobile emulation or a different scale setting.
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The image is taller or shows different sections
Check capture scope. A viewport screenshot and a full-document screenshot are different products. If a full-page API is unavailable or behaves differently for your browser, scroll-and-stitch logic may be required, but that introduces its own seams and timing concerns. Prefer a documented full-page method when your browser binding supplies one.
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Only text or thin lines differ
Confirm browser/driver builds, operating system or container, scale factor, and font availability. The documented controls do not establish one universal cause for font or compositor differences, so preserve both manifests and investigate those host variables rather than changing random CSS.
The layout differs despite matching dimensions
Compare page data and readiness. Late network responses, animations, lazy images, and font swaps can change layout after navigation reports success. Wait for the relevant application condition and capture at the same lifecycle point.
The command launches, but the result is unexpected
Check the exact Headless implementation and Chrome version. Chrome’s unified Headless behavior and the standalone old shell have different version boundaries. Ensure the driver matches the browser and save the complete startup command for reproduction.
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This compact script sets a size, navigates, records measurements, waits for a page condition, and saves a viewport screenshot. Adapt the URL and condition to your application:
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import json
from selenium import webdriver
from selenium.webdriver.chrome.options import Options
from selenium.webdriver.support.ui import WebDriverWait
URL = "https://example.com"
options = Options()
options.add_argument("--headless=new")
options.add_argument("--window-size=1280,900")
driver = webdriver.Chrome(options=options)
try:
driver.set_window_size(1280, 900)
driver.get(URL)
WebDriverWait(driver, 30).until(
lambda d: d.execute_script("return document.readyState") == "complete"
)
WebDriverWait(driver, 30).until(
lambda d: d.execute_script("return document.fonts ? document.fonts.status === 'loaded' : true")
)
manifest = {
"window": driver.get_window_rect(),
"metrics": driver.execute_script("""
return {innerWidth: innerWidth, innerHeight: innerHeight,
devicePixelRatio: devicePixelRatio,
visualViewport: visualViewport ? {
width: visualViewport.width, height: visualViewport.height,
scale: visualViewport.scale} : null};
"""),
"capabilities": driver.capabilities
}
print(json.dumps(manifest, default=str, indent=2))
driver.save_screenshot("diagnostic.png")
finally:
driver.quit()
Selenium’s Chromium API reference documents the Chromium WebDriver methods. Treat this script as a diagnostic baseline, not a promise of identical pixels across different machines.
Or skip the browser setup
ScreenshotNeo is a website screenshot API and MCP server when you need a rendered URL without provisioning Selenium. One GET request returns PNG, JPEG, WebP, or PDF. Before capture it accepts the cookie or consent banner like a visitor and removes more than 60 known consent platforms, newsletter popups, and chat widgets; each step can be disabled. Bot checks or CAPTCHAs, blank pages, timeouts, failed loads, and cache hits are not billed, and response headers report the page verdict and billing status.
It also supports full-page and CSS-selector captures, dark mode, 12 device presets or custom viewports, retina scale, PDF paper and page-range controls, custom CSS and JavaScript, pre-capture clicks, selector waits, delays, network-idle waits, request and resource blocking, headers, cookies, user agents, Authorization, timezone, geolocation, transparent backgrounds, resizing, chosen cache TTLs, signed image links, asynchronous webhooks, 100-URL bulk calls, a usage API, and an OpenAPI specification. Its MCP server provides take_screenshot, get_page_info, and capture_pdf tools for Claude, Cursor, and other MCP clients.
cURL:
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpPython:
import requests
r = requests.get("https://api.screenshotneo.com/v1/shot", params={"access_key": "YOUR_API_KEY", "url": "https://stripe.com"}, timeout=90)
open("shot.webp", "wb").write(r.content)Node.js:
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Cost, reliability, and reproducibility notes
- Keep browser and driver binaries pinned or deliberately upgraded together, and record upgrades in the comparison artifact.
- Cache diagnostic manifests and images as CI artifacts; a failed visual assertion without its environment is difficult to explain.
- Use deterministic test data and disable or control animations where the product permits it.
- Separate a viewport-regression test from a full-page-document test; they answer different questions.
- When a mismatch remains, change one variable at a time: viewport, scale, implementation, host, then page timing.
FAQ
Does --headless=new guarantee the same pixels as headful Chrome?
No. It selects the current unified implementation, but viewport, scale, host, fonts, compositor behavior, and page timing still need to be controlled and measured.
Should I compare PNG dimensions or CSS dimensions?
Both. CSS dimensions describe layout; bitmap dimensions also reflect device-pixel ratio and screenshot scaling. Log both before comparing pixels.
Is a full-page screenshot always better for regression testing?
No. Use full-page capture when document coverage is the requirement. Use viewport or element capture when the test concerns a defined visible region; the correct scope is the one that matches the assertion.
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